The Moloney murine sarcoma virus (MSV)-related normal cell sequence c-mos, cloned from Balb/c mouse cells, is identical to the MSV-specific transforming sequence v-mos, but is unable to transform cells in DNA transfecton assays. Mos is the specific transforming sequence carried by the Moloney MSV. V-mos refers to that sequence found in the viral genome, while c-mos is its cellular homology found in normal mouse cells. The transforming potential of c-mos was activated by linking it to the MSV long terminal repeat (LTR) sequence derived from a recombinant DNA clone containing the whole MSV provirus. The LTR is a 600 base pair sequence which contains presumptive RNA control sequences ad which is repeated at both the 3' and 5' termini of the provirus. We are characterizing the nature of mos-containing RNA expressed in these transformed cells. We are attempting to activate the expression of other transforming sequences by linking the viral LTR to these sequences and measuring their ability to transform normal mouse cells.